Custom extension for not-yet-complete downloads

what I've in mind is something like how Firefox handles downloads.
By browsing the filesystem you can see if a file hasn't yet finished downloading since it ends with a .part extension.
When the download is finished the additional extension is removed.

This would be extremely handy for a FTP application to have.
Indeed, I often find myself queueing many small files for download on a single directory and would like to be able to open a file every now and then even if the queue isn't empty.
Without this feature I have to manually check, which can easily become irritating, if the file has already been downloaded or if it's still downloading.

This feature could be optional, maybe at the favorite level.
Many other download applications have this feature, FlashGet and OrbitDownloader come to mind.

If you use multi part downloads the .part file is created. Otherwise is not. In this case SmartFTP also saves a file with the section information. Doing this for every download does not really make sense in my opinion.